Remote characterization of the rheological properties of sludges and slurries - Preliminary investigations into a novel in situ sensor

Multiphase process design and optimization often depends upon the rheological properties of sludges and slurries. In practice the task of determining these important properties is usually carried out by removing small samples from a process line or process unit and performing tests with proprietary bench-top analysers near to site or at a remote laboratory. Sampling will remain an important procedure in industrial processes, but there is also a strong case for augmenting the routine information obtained using an in situ sensor, as suggested below. Sampling protocols can result in extensive turnaround periods from sample collection to the generation of results. The delay can have an effect on a variety of factors such as product quality, process operability and efficiency. Sampling of toxic or radioactive material can prove an even greater challenge to minimize human involvement. Removal often changes the rheology of a specimen, and might lead to incorrect conclusions being drawn about the condition of a sludge. Sludge and slurry in a containment suffers a gravitational gradient, so that a removed sample should be paired with its location, particularly in the vertical dimension. Sludge or slurry removed from a duct or from siphoning tubes suffers a velocity gradient such that lighter fractions are preferentially represented in the sample, causing an underestimate of coarser particles. Many more data points can be collected by a remote instrument, improving the sampling statistics of the measurement. This paper presents results of an investigation using a small piezo-electric driven AT-cut quartz crystal to measure changes in suspension stability. Mono-dispersed silica suspensions were prepared in several different chemical environments, with the conditions chosen from examination of the zeta potential and sedimentation data. The study shows that by varying the suspension stability, the resonating frequency of a 5 MHz AT-cut quartz silica crystal significantly deviates as the amplitude of oscillation is increased for a coagulated suspension, while remaining unchanged for a dispersed suspension.